[Image above] Credit: Bentaylor13413, Wikimedia (CC BY-SA 4.0)
Viscosity is a measure of a fluid’s resistance to flow. Water flows readily, having a low viscosity, whereas honey has a high viscosity and flows considerably more slowly.
Viscosity changes dramatically with temperature, increasing as the fluid is cooled due to decreasing thermal energy at the atomic level. When cooled below its liquidus temperature, normally a liquid will solidify through crystallization into regular structures with low equilibrium energy.
However, as glass forming systems cool, they do not follow the equilibrium crystallization path. Instead, glass systems freeze in the disordered atomic structure of the supercooled liquid with extremely high viscosity. In the nonequilibrium glassy state, atoms continue to move and relax towards equilibrium.
The transition from liquid to glassy systems and vice versa is measurable through changes in viscosity, volume/density, enthalpy/heat capacity, and other properties. This transition is not sharp and distinct like melting and vaporization. Rather, the glass transition occurs over a range of temperatures, dependent on cooling rate and holding temperatures and times (in the glassy condition).
To capture this effect of thermal history, glass scientists define fictive temperature (Tf) as the point of intersection of the extrapolated properties of the liquid and the glass. Fictive temperature is used as a benchmark for modeling variations in structure and properties of glasses.
Viscosity and fictive temperature are the topics of this month’s Glass: Then and Now series, which celebrates the International Year of Glass. We asked John Mauro to provide insights into the importance of these issues. Mauro is the Dorothy Pate Enright Professor and Associate Head for Graduate Education at Penn State University, an ACerS fellow, and incoming editor-in-chief for Journal of the American Ceramic Society.
Viscosity is one of the most important properties of glass-forming systems, governing every step of industrial glass production. Industrial glass melting depends on the molten glass having sufficiently high fluidity to homogenize and remove bubbles. After melting, glass forming operations are all designed to occur at specific viscosities, where the formability of the glass must be balanced against the need for the glass product to hold its shape while cooling.
Hence, the temperature and composition dependence of viscosity is also of great scientific interest, posing a century-long challenge for glass scientists. The viscosity of a supercooled liquid increases by orders of magnitude as it is cooled, eventually leading to a falling out of equilibrium, i.e., the glass transition, where the degree of disequilibrium is described by the fictive temperature.
This month, we highlight two classic papers on viscosity and fictive temperature. Fulcher’s classic 1925 paper introducing the Vogel-Fulcher-Tammann (VFT) equation for viscosity is one of the most highly cited papers in the history of Journal of the American Ceramic Society. Ritland’s 1956 paper proposed the classic cross-over experiment, repeated seven years later by Kovacs in the polymers community, which turned a critical eyes to the concept of fictive temperature itself. Finally, we highlight two recent articles that provide the latest thinking on both fictive temperature and viscosity. Over this past century, Journal of the American Ceramic Society has remained at the forefront of research in both these areas.– John Mauro, Penn State
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